Film manufacture



Oct. 3, 1967 v D. M. DAvis ETA-L 3,344,474

FILM MANUFACTURE Filed Oct. 29. 1963 United States Patent 3,344,474 FILM MANUFACTURE Donald M. Davis, Claymont, Del., Anthony J. Schreiber, Philadelphia, Pa., and Harry W. Sherman, Wilmington, DeL, assignors to Avisun Corporation, Philadelphia,

Pin, a corporation of Delaware Filed Oct. 29, 1963, Ser. No. 319,719 7 Claims. (Cl. 1815) ABSTRACT OF THE DISCLOSURE In the manufacture of film by extruding molten material into a quenching bath, quenching liquid is continuously withdrawn from along the path of the extruded stream of film-forming material. The withdrawal is effected by slotting one or more of the guide tubes over which the film passes in the bath, and connecting a suction pump to the slotted tube. The slotted tube may be one of the rolls of a tension frame.

The present invention relates to the manufacture of films or webs having improved optic-a1 or physical properties.

In forming continuous films or webs by conventional procedures a shaped stream of molten film-forming material is extruded downwardly into a bath of cool water or other liquid while it is quenched into a solid film or web. This formed film is then removed from the quenching bath and generally subjected to further processing.

One particular objection to this known method is that the resulting film or web often exhibits longitudinally extending and usually discontinuous striations or streaks 'which not only detract from its appearance but also make further processing, such as uniaxial or biaxial stretching, difficult if not impossible. While many prior proposals have been made for remedying this problem none has proved to be adequate. Accordingly, a primary object of this invention is to provide a new or generally improved and more satisfactory method and apparatus for making continuous films.

Another object is the provision of a method of making thermoplastic films having substantial-1y uniform optical and physical properties throughout their entire lengths.

Still another object is the provision of a method and apparatus for producing films from thermoplastic materials which are substantially free of longitudinal striations or streaks.

Still further objects will appear from the following description.

The method and apparatus of the present invention is based upon our discovery that during the quenching of a shaped stream of molten film-forming material within a liquid bath minute gas bubbles are formed within the quenching liquid and that these bubbles often ride along one or both surfaces of such shaped stream as they rise to and escape from the bath surface. More particularly, it is believed that a thin layer of air, and possibly other gases, are carried by the shaped stream of film-forming material as it enters into the bath of quenching liquid. So long as these air layers travel along with the shaped stream of film-forming material, no particular difficulties are encountered in either the method or the resulting film.

We have dis-covered, however, that once the shaped stream of film-forming material or the resulting film is engaged with some fixed structure within the quenching bath, as for example a guide or tension bar, the layer of air on that side of the film is disrupted and dispersed as a mass of tiny bubbles. Some of these air bubbles rise immediately to the bath surface. The remaining bubbles generally collect on the submerged structure and periodically breakaway and escape into the surrounding atmosphere, as for example when the quenching bath is agitated.

In rising to the surface of the quenching bath, certain of the air bubbles appear to ride along the adjacent surface of the shaped stream of film-forming material and, in effect, insulate such stream from the quenching liquid. While such air bubbles provide for only a momentary shielding of the shaped stream of film-forming material, the effect of the differential cooling which results is well apparent in the finished film. Further, while the area of the shaped stream of film-forming material which is momentarily insulated by any one bubble is small the total area of the shaped stream which is covered by such moving bubble is significant and appears as a longitudinal- 13 extending striation or streak in the finished film. Additionally, the fact that the air bubbles break-away from the submerged fixed structure at no set time intervals accounts for the generally discontinuous nature of the film striations.

In the film produced by the above described conventional method, the striated areas may be more or less transparent than the remaining areas so that the film presents an undesirable appearance. More important, and as heretofore mentioned, when such film is stretched in either a uniaxial or biaxial direction the different areas of the film do not yield at the same rate or to the same degree so that uniform molecular orientation is difficult, if not impossible to achieve.

In accordance with our invention, the above-noted objections are avoided by preventing gas bubbles from riding along the surfaces of a freshly extruded stream of filmforming material during the quenching thereof into a solid film or web. More particularly, in view of our discovery as to how and where such gas bubbles are formed we achieve the above objective by having hollow guide or tension bars, or other fixed structures which are submerged within the quenching bath and with which the resulting film first engages, serve to continuously withdraw quenching liquid from along the path of the extruded stream of film-forming material. In this manner the layers of gas which are carried by such streams or the gas bubbles which are formed in the area of such guide or tension bars and which tend to move against or toward the quenched film are also removed with the withdrawn bath liquid before they can do any damage.

Essentially, the guide or tension bars employed in the apparatus of the present invention are in the form of tubes, each having an elongated discharge slot which, for most efiicient operation, must be located close to but out of contact with the adjacent film surface. The guide or tension bars are operatively connected to a suitable pump or other suction means. The application of excess suction to the guide or tension bars should be avoided to prevent the introduction of excess turbulence into the quenching liquid.

Cool liquid is continuously supplied to the quenching bath to compensate for that withdrawn through the guide or tension bars. In addition to achieving the intended purpose of removing objectionable gas bubbles from the quenching bath, it has been found that the continuous circulation of the bath liquid avoids the formations of hot spots along the path of the shaped stream of film-forming material. As a result, more efficient quenching is effected and a greater assurance is provided that all portions of such stream are subjected to substantially the same quenching conditions.

In the drawing, FIGURE 1 is a fragmentary end view of a portion of one embodiment of the apparatus of the present invention;

FIGURE 2 is a view similar to FIGURE 1 on an enlarged scale;

FIGURE 3 is a view similar to FIGURE 1 showing a modified embodiment of the apparatus of the present invention; and

FIGURE 4 is a side view of the structure shown in FIGURE 1 as viewed along the line IVIV of FIG- URE 3.

With reference to FIGURES 1 and 2 of the drawing, 11 indicates a conventional nozzle which is designated to extrude a continuous and generally flat stream 13 of film-forming material, as for example molten polyethylene or polypropylene. A quenching or coagulating liquid 15, (see FIGURE 3), such as cold water, is contained within a tank 17 and serves to chill or set the shaped stream 13 into the form of a film or web 19. The tank 17 is of known construction and includes a roller 21 for reversing the path of the film 19 and directing the same to a roller 23 which assists in advancing the film to a collecting means, not shown.

At least one and preferably a pair of hollow guide bars or tubes 25 are positioned within tank 17 to minimize deflection of the film 19 from its intended path of travel and to assist the roller 21 in reversing the direction of film travel. The guide bars 25 are each provided with a slot 27, which extends across the entire width of the film 19, and are connected by conduits 29 to a suitable pump or other suction means, not shown. During operation, the liquid bath 15 is continuously withdrawn from opposite sides of the film 19 through the slotted bars 25 and is replaced by cool liquid, as supplied by conduits 31. Baflies 33 are positioned between the shaped stream of film-forming material 13 and the liquid supply conduits 31 to suppress excessive fluctuations along that portion of the bath 15 through which the stream 13 first passes.

With the above described apparatus it will be apparent that the slotted guide bars 25 and the liquid supply conduits 31 together provide for continuous circulation of the bath liquid along the opposite sides of the film bath. As a result, stagnant areas are eliminated and substantially all portions of the continuously extruded stream of film-forming material 13 are subjected to substantially the same quenching conditions. Mort important, gas bubbles which might otherwise ride along the sides of the shaped stream of film-forming material 13 and induce the formation of striations or streaks are continuously removed from the bath liquid. In the attainment of both of these objectives, care must be exercised to avoid the application of such suction to the slotted guide bars 25 as to create excessive turbulence within the bath along the opposite sides of the shaped stream 13.

The embodiment of the invention shown in FIGURES 3 and 4 differs from that described above by employing a tension frame or wiper bar assembly 35 which is pivotally supported by brackets 37 in the vicinity of, but below, the surface of the bath 15. The tension frame 35 is similar in construction and function to that described in the United States patent application of A. J. Schreiber and H. W. Sherman, Ser. No. 283,463, filed May 27, 1963 (now Patent No. 3,245,117, dated Apr. 12, 1966), and includes at least one and desirably a pair of hollow guide bars or tubes 39 and 41, which are supported between trunnions 43, and a pin (not shown) for locking the frame in adjusted position.

The guide bars 39 and 41 of the specific embodiment illustrated are designed to minimize thickness variations across the width of the shaped stream 13 of film-forming material which, when extruded, normally exhibits thicker longitudinal edge portions. To achieve this purpose the bars 39 and 41 are each formed with a center section 47 which is substantially cylindrical and outwardly tapered end sections 49. As illustrated, at least the uppermost guide bar 39 is provided with an elongated slot 51, which extends across the entire width of the film 19, and is connected by a flexible conduit 53 to a suitable pump or other suction means, not shown.

At the start of operations, the stream of film-forming material 13 is extruded downwardly between the guide bars 39 and 41 after which the frame 35 is adjusted into a position as shown in FIGURE 3. The tapered end sections 49 of the respective guide bars cause the edge portions of the film 19 which are engaged therewith to travel at a more rapid rate of speed than the remainder of the film. As a result, longitudinal tensions are transmitted rearwardly and react upon the yieldable or plastic edge portions of the shaped stream of film-forming material 13 to thereby stretch the same. The edge portions of such stream 13 become thinner as they are stretched so that thickness variations between the center and edges of the stream are eliminated or minimized before actual quenching or setting takes place.

The slotted guide member 39 of the tension frame 35 operates in substantially the same manner as the guide bars 25 heretofore described. More particularly, the slotted guide bar 39 serves to continuously remove the bath liquid from the adjacent side of the film 19 and in so doing removes any gas bubbles which might otherwise tend to ride along such film and induce the formulation of striations or streaks therein. Generally, bubbles which are formed in the area of the lowermost guide bar 41 rise to the surface of the bath 15 without hindering the quenching of the shaped stream 13. Thus, while it is not essential that such lowermost guide bar 41 serve as a bath discharge means, it may be also formed of slotted construction, in substantially the same manner as the bar 39, to further limit the possibility of film damage by gas bubbles and to provide for more eflicient bath circulation.

It will, of course, be apparent that the rate of speed at which any particular portion of the film is advanced is directly proportional to the diameter of the section of the guide bar with which such film portion is engaged. Thus, when extruding streams of film-forming material having different cross-sectional dimensions than that of the stream described above, the spacing of the guide bars 39 and 41 must also be varied to obtain the desired stretching of the thicker portions of such streams. For example, with a stream of film-forming material having a central portion which is thicker than the edges thereof, the guide bars 39 and 41 would have a longitudinal section of double convex construction.

It is to be understood that changes and variations may be made without departing from the spirit and scope of the invention as defined in the appended claims.

We claim:

1. Apparatus for making a continuous film comprising means for extruding a continuous, generally flat stream of film-forming material, a tank containing a liquid bath for setting the extruded stream into a film, means for advancing the film through and outwardly from said tank, a tube positioned within said tank for engaging with one side of the film, said tube having an opening therein; and suction means connected to said tube for removing bubbles of gas from at least one side of the film as it approaches said tube.

2. Apparatus as defined in claim 1 wherein said tube opening is a slot which extends across the width of the film being formed, and wherein the tube is positioned to direct said slot toward but out of contact with the adjacent side of the approaching film.

3. Apparatus as defined in claim 2 including also a second slotted tube positioned on the opposite side of the film and connected to said suction means whereby bubbles of gas are removed from opposite sides of the approaching film.

4. Apparatus for making a continuous film comprising means for extruding a continuous, generally flat stream of film-forming material, a tank containing a liquid bath for setting the extruded stream into a film, means for advancing the film through and outwardly from said tank, a tension frame having at least one guide bar which extends across the width of the film, said frame being adjustably supported by said tank below but in the vicinity of the surface of the liquid bath, whereby said guide bar may be snugly engaged by the film to minimize deflections thereof by movement of the liquid bath, said guide bar having therein an elongated slot extending along the length thereof, said slot being directed toward the path of the advancing film; and suction means connected to said slotted guide bar for preventing gas bubbles from forming and gathering on the adjacent surface of the film.

5. Apparatus as defined in claim 4 wherein said guide bar is provided with an enlarged section for causing the portion of the advancing film which is engaged thereby to travel at a faster rate of speed than the remainder of the film, whereby the portion of the extruded stream which is longitudinally aligned with such portion of the film is stretched and reduced in thickness.

6. Apparatus as defined in claim 4 wherein said tension frame includes two guide bars which are adapted to engage with opposite sides of the film when the frame is moved into adjusted position, and wherein at least the uppermost of said guide bars is slotted.

7. Apparatus for making a continuous film having improved optical and gauge characteristics comprising means for extruding a continuous stream of molten thermoplastic material having enlarged edge portions, a tank containing a liquid bath for quenching the extruded stream into a film, means for advancing the film through and outwardly from said tank, a tension frame adjustably supported within said tank below the surface of the liquid bath, said tension frame including a pair of guide bars which are adapted to engage with opposite sides of the film when the frame is in adjusted position to thereby restrain the film and extruded stream against deflection by movement of the liquid bath, said bars each having a central section of substantially uniform diameter and end sections which taper outwardly to an enlarged diameter, said tapered end sections being adapted to engage with the enlarged edge portions of the film and to cause the same to travel at a more rapid rate of speed than the remainder of the film; at least the uppermost of said pair of guide bars having therein a slot which extends across the width of the film which is formed; and suction means operatively connected with said slotted guide bar for continuously withdrawing the liquid bath from one side of the film as it approaches such guide bar.

References Cited UNITED STATES PATENTS 2,800,682 7/ 1957 Dooley 264-23 2,987,373 6/ 1961 Bezemer 264-178 3,029,124 4/ 1962 Farrow 264l78 3,087,198 4/ 1963 Edwards 18-15 3,108,324 10/1963 Zavasnik 264210 3,199,148 8/1965 Koppehele 18-15 3,245,117 4/ 1966 Schreiber et a1 18l5 WILLIAM J. STEPHENSON, Primary Examiner.

D. J. ARNOLD, Examiner. 

1. APPARATUS FOR MAKING A CONTINUOUS FILM COMPRISING MEANS FOR EXTRUDING A CONTINUOUS, GENERALLY FLAT STREAM OF FILM-FORMING MATERIAL, A TANK CONTAINING A LIQUID BATH FOR SETTING THE EXTRUDED STREAM INTO A FILM, MEANS FOR ADVANCING THE FILM THROUGH AND OUTWARDLY FROM SAID TANK, A TUBE POSITIONED WITHIN SAID TANK FOR ENGAGEING WITH ONE SIDE OF THE FILM, SAID TUBE HAVING AN OPENING THEREIN; AND SUCTION MEANS CONNECTED TO SAID TUBE FOR REMOVING BUBBLES OF GAS FROM AT LEAST ONE SIDE OF THE FILM AS IT APPROACHES SAID TUBE. 